1. Field of the Invention
The present invention relates to an optical pickup apparatus and an optical disc apparatus, and particularly to an optical pickup apparatus and an optical disc apparatus allowing a small-sized apparatus to perform stable aberration correction.
2. Description of Related Art
In recent years, in an optical disc apparatus typified by a Digital Versatile Disc (DVD) recorder/player and the like, further downsizing and reduction in thickness have been desired for adaptation to mobile equipment such as a laptop personal computer and a mobile DVD player, and demand for space saving, and so on. Also, in a mobile game machine and the like, an optical disc apparatus capable of reproducing a high-density disc such as a Universal Media Disc (UMD), and which can be manufactured so as to have a small size and a small thickness has been needed. Furthermore, in an apparatus using a high-density next-generation optical recording medium typified by a Blu-ray disc (trademark), downsizing and reduction in thickness are unavoidable in order to realize higher-performance, improvement in design and the like,
In order to realize such a smaller and thinner optical disc apparatus, downsizing and reduction in thickness of an optical pickup employed in the optical disc apparatus and its optical system are required.
In the past, in an optical disc apparatus using a recording medium whose NA is not so high (that is, not requiring a high storage capacity), such as a Compact Disc (CD) and a Mini Disc (MD), an optical pickup employing a so-called infinite optical system as shown in FIG. 1 is changed to a pickup employing a so-called finite optical system as shown in FIG. 2 to achieve downsizing, and reduction in thickness of the apparatus (for example, refer to Japanese Patent Application Publication (KOKAI) No. 2005-247034).
In the infinite optical system, a light irradiated from a light source 11 is changed to a parallel light through a collimator lens 12, and passes through an objective lens 13 to thereby be collected on a recording surface of a recording medium 14.
The infinite optical system is configured such that no aberration occurs with respect to shift of the objective lens 13 corresponding to tracking servo or focusing servo. However, in the infinite optical system, a distance from the light source 11 to the collimator lens 12 needs to be almost the same as a focal distance of the collimator lens 12, and an outer diameter dimension and a light flux diameter of the collimator lens 12 need to be so large that a parallel light flux thick enough to satisfy a movement range of the objective lens 13 can be obtained, which limit downsizing and reduction in thickness of the entire optical system.
In contrast, in the finite optical system, the light irradiated from the light source 11 arranged at a finite distance from the objective lens 13 is directly received by the objective lens 13, or is received by the objective lens 13 through a coupling lens 15 to be collected on the recording surface of the recording medium 14.
In the finite optical system, since no lens other than the objective lens 13 may be used, or the coupling lens 15 having a relatively small lens outer diameter can be used, and further, a distance between an object and an image can be short, downsizing and reduction in thickness of the entire optical system can be achieved. While aberration with respect to the shift of the objective lens 13 for tracking servo or focusing servo occurs, in a CD or MD of a low NA, the aberration amount caused by the shift of the objective lens is not so large that the system margin is collapsed, and thus, downsizing and reduction in thickness by the finite optical system have been allowed to be realized.